1. Field of the Invention
The present invention relates to a discharge-pumped gas laser with preionizers, and more particularly to a high-repetition-rated discharge-pumped gas laser which is capable of developing a stable electric discharge between main discharge electrodes through the control of a laser gas flowing between the main discharge electrodes and a laser gas flowing through preionizers.
2. Description of the Prior Art
Discharge-pumped gas lasers, particularly discharge-pumped rare-gas-halide excimer lasers, can be excited for highly efficient high-output laser emission at a high repetition rate. For this reason, the discharge-pumped gas lasers are used as laser emission sources in a wide variety of applications including research and development activities, semiconductor fabrication processes, chemical engineering processes, materials processing applications, and medical applications among other fields.
Some conventional discharge-pumped gas lasers have ultraviolet preionizers which cause a spark discharge to preionize the laser gas in a main discharge region between confronting main discharge electrodes. The laser gas is deteriorated because it contains ions and metal particles emitted from the preionizers due to sputtering upon the spark discharge. When a deteriorated laser gas remains present in the main discharge region between the main discharge electrodes, the electrons produced by the preionization of the laser gas are not spatially distributed uniformly in the main discharge region. Therefore, the main discharge current flowing through the main discharge region between the main discharge electrodes is localized, tending to produce an arc discharge and result in low laser emission efficiency. As a result, the discharge-pumped gas lasers emit laser beams of relatively small spot size and low output level, and the main discharge electrodes have a short service life.
Furthermore, the electrode pins of a preionizer located downstream of the main discharge region tend to be heated and damaged by a deteriorated high-temperature laser gas containing ions and metal particles emitted from the main discharge electrodes. The electrode pins of the downstream preionizer are worn quickly particularly during repetitive operation of an excimer laser. In a KrF excimer laser, for example, the electrode pins of the preionizer that are heated to high temperature and the fluorine gas rapidly react with each other, and hence the electrode pins are worn and the fluorine gas are consumed at a high rate. Consequently, it is difficult to accomplish stable laser emission at a high repetition rate in such an excimer laser.
In order for a discharge-pumped gas laser to emit a stable laser beam with high laser emission efficiency, it is necessary that no deteriorated laser gas remain present in the main discharge region. To meet this requirement, the laser gas should be circulated at high speed through the main discharge region as a laminar gas flow. Generally, the repetition frequency f (Hz) of laser emission and a minimum laser gas speed V (m/sec.) that is necessary are related to each other according to the following equation: EQU V=f.times.L.times.CR
where L is the distance (m) between the gap of the preionizer, which is located upstream of the main discharge region, and the downstream end of the main discharge electrodes, and CR is the clearing ratio which has a value of 1 when the laser gas flow is a complete laminar flow and a larger value as the laser gas flow becomes more turbulent. It is practical for one conventional discharge-pumped gas laser design to have the distance L of 0.06 m and the clearing ratio CR of about 3.
For such a conventional discharge-pumped gas laser to achieve a high repetition rate of 500 Hz, the laser gas is required to flow at a speed of 90 m/sec. or higher. Such a high-speed laser gas flow can be achieved by a large-capacity gas circulator, resulting in a large gas laser size.